We're guessing the contest, which benefits the nonprofit Water Defense, got an extra boost when Ruffalo tweeted a photo of himself on the set with director JossWhedon.

Prizeo says Ruffalo's team had to seek permission from Marvel Studios and Disney to clear a day for the visit, which will take place in mid-July. Though the contest is lasting just days, Prizeo and Ruffalo's camp had been discussing it for months.

"We've never done a campaign so short before. But how can we say no to someone so passionate?" Prizeo head of promotion Candace Cheong tells The Hollywood Reporter. "He's also one of the nicest people in the universe. I'm sure he'll show the winner a really good time."